It now has about 390 hours on it, and last weekend after riding for about three hours, we were on our way back to the dock, running about 3000rpm, and I lost power (felt like I hit a sand bar). My first reaction is always to turn the key off. After checking depth, I tried to start the boat back up and it would not start.

It took a lot of gas to get it started and we noticed a very small amount of white smoke out the exhaust. It would not idle, or run below 1200 rpm. I was able to get us back to the dock (another 1/4mile or so) and turned the boat off.

Waited about 10 minutes for my trailer and started it back up again, same thing, still a small amount, but more white smoke, and we noticed a small amount of oil slick behind the boat as well.

I just pulled the plugs today and they look like this (check pics below):

Any ideas? I have no idea where to even start. My compression gauge only fits on two of the spark plug holes because the exhaust manifolds are in the way, but the two that I checked were both at 135psi.

The plugs do not look fouled, except there seems to be one or two that have a film of oil on them. It is common for the plugs to get a dark haze on them, but a sooty black coat is definetely fouled.

To clear a flood you open the throttle 3/4 too full throttle. This takes the vacuum away from the jets, which equals less fuel. The point here is that you are dumping fuel into the motor. There are only two things that cause this problem. I assume you have a Holley carb?? Float level can cause a major problem. The second being a power valve. I have replaced alot of power valves on Holleys. In fact I did my own just to be safe that it would not blow out on me. How old the carb is does not matter and backfiring is not always the cause. There is no check for a bad power valve other than tear down and I would just replace it anytime a float bowl on a Holley is opened.

Where to go from here? Well......keep posting any finds or facts that you have and I will see what I can do to help. I highly doubt it is an engine mechanical problem.

I agree with Peter. It sounds like you may have blown a power valve. But to be safe. Drain some of your oil and look at it. If it's milky then you have a blown head gasket. But it doesn't sound like that's the problem. Just do it to be sure.

Your plugs look like your air/fuel ratio is pretty rich. But that could have all been from the little bit of running time with a ruptured power valve just getting back to the dock.

Oil on the spark plug threads is usually from valve seals leaking a little bit. It doesn't look like it's very bad on your plugs though. Changing valve stem seals is pretty easy if that is the case.

You can not check the valve seals. The test for them is start the motor up and see if it burns oil. Unless they are completely shot and broken you can not tell how hard or worn the seals are. A sticking oil ring can cause oil to get into the combustion chamber. Non of this is your most pressing running problem as they would just cause a cylinder or two too miss not a major runability problem.

As for water in the oil it will be the first thing to drain out of the oil pan if that would be the case. If you are not due for an oil change and it is still clean you can just undo the end plug and quickly see what comes out then recap it.

I put the plugs back in, but now that I am thinking about it... It is possible that the plugs fouled (while running for 90 hours with that bad carb), and unburned gas mixed with the carbon/soot deposits on the plugs, and it just looked like oil. The rainbow I saw behind the boat could have also just been unburned gas coming out the exhaust due to the fouled plugs. Let's hope so!!

Those plugs you have in there are great spark plugs, I ran them in my Ford motor. They are GM's special marine spark plugs, that is what the "MR" stands for. Motorcrafts are good plugs too but will rust. The GM's are stainless. Autolites are complete crap! Never ever use them in anything.....or so my opinion goes.

The rainbow sheen on the water definetely could have been gas, from the engine dumping fuel. As for the plugs being fouled from the previous carb it is unlikely as they will clean themselves once the motor is running correctly.

I can not tell from the pictures. Are a couple of the plugs covered in oil? They will be shiny.

My guess is powervalve too. I had the same problem this summer. Did you use a rebuild kit for a Holley that was not marine specific? If so, the powervalves are not the same. The marine unit is shorter and has a different vacuum pressure to open.